FIG. 1 shows interworking architecture of a 3rd Generation Partnership Project (3GPP) packet switched (PS) network and a CDMA2000 1x network in a code division multiple access (CDMA) network in the prior art. Referring to FIG. 1, the architecture includes: an evolved UMTS terrestrial radio access network (E-UTRAN), which is adapted to implement all radio-related functions of the evolved network; a mobility management entity (MME), which is responsible for the mobility management of the control plane, including user context management and mobility state management as well as assignment of temporary user IDs; a serving gateway (SGW), which is a user plane anchor point between 3GPP access networks, and adapted to terminate an interface of the E-UTRAN; and a packet data network gateway (PGW), which is a user plane anchor point between a 3GPP access network and a non-3GPP access network, and adapted to terminate an interface with an external packet data network (PDN).
A UMTS terrestrial radio access network (UTRAN) and a GSM/EDGE radio access network (GERAN) are adapted to implement all radio-related functions in an existing GPRS/UMTS network. A serving GPRS supporting node (SGSN) is adapted to implement routing and forwarding, mobility management, session management, user information storage, and other functions in the GPRS/UMTS network.
A 1x circuit switched (CS) domain access network is adapted to implement radio-related functions in the CDMA2000 1x network. A 1x CS mobile switching center (MSC) is mainly adapted to provide mobility management and switching functions for mobile users, and realize the interworking between mobile users as well as between mobile users and fixed users. An interworking solution function (IWS) is adapted to implement a function of exchanging and forwarding 1x CS signaling, so as to encapsulate 1x CS signaling from the 1x CS MSC and send the encapsulated 1x CS signaling to a user equipment (UE) through the 3GPP PS network; or the UE sends encapsulated 1x CS signaling to the IWS through the 3GPP PS network, and the IWS forwards the encapsulated 1x CS signaling to the 1x CS MSC. The IWS is also described as a 1x CS IWS.
In order to ensure the interworking between the 3GPP PS network and the original CS network, a technology for handing over a voice over IP (VoIP) service in the 3GPP PS network to the CDMA2000 1x network has been proposed in the prior art. FIG. 2 is a signaling flow chart of a procedure for handing over a VoIP service of a UE from a 3GPP PS network to a CDMA2000 1x network in the prior art. Referring to FIG. 2, the procedure is as follows:
1. A VoIP session exists in the 3GPP PS network;
2. An access network of the 3GPP PS network decides to hand over to the CDMA2000 1x. The access network of the 3GPP PS network may be an E-UTRAN, a GERAN, or a UTRAN;
3. The access network of the 3GPP PS network sends a relocation indication to notify the UE to perform an inter-system handover;
4. The UE sends a 1x Origination message to a 1x CS IWS through the 3GPP PS network. That is, the UE sends the 1x Origination message to the access network of the 3GPP PS network, and the access network of the 3GPP PS network encapsulates the 1x Origination message into an Uplink CDMA2000 Tunnel message and sends the Uplink CDMA2000 Tunnel message to a mobility management network element of the 3GPP PS network. For the GERAN/UTRAN network, the mobility management network element of the 3GPP PS network is an SGSN; and for the E-UTRAN, the mobility management network element of the 3GPP PS network is an MME. After receiving the Uplink CDMA2000 Tunnel message, the mobility management network element of the 3GPP PS network acquires the 1x Origination message from the Uplink CDMA2000 Tunnel message, and sends the 1x Origination message to the 1×CS IWS;
5. The IWS sends a CM Service Request message to a 1x CS MSC;
6. The MSC sends an Assignment Request message to the IWS;
7. The IWS sends a Handoff Direction message to the UE through the 3GPP PS network. That is, the IWS sends the Handoff Direction message to the mobility management network element of the 3GPP PS network; the mobility management network element of the 3GPP PS network sends a Downlink CDMA2000 Tunnel message (the Handoff Direction message is encapsulated in the message) to the access network of the 3GPP PS network; and the access network of the 3GPP PS network forwards the Handoff Direction message to the UE;
8. The UE is handed over to the CDMA 1x network, and a traffic channel acquisition (TCC) procedure is performed to obtain a traffic channel;
9. The UE sends a 1x Handoff Complete message to the CDMA2000 1x access network;
10. The CDMA2000 1x access network sends a Handoff Complete message or an Assignment Complete message to the MSC;
11. A voice call is established in the CDMA2000 1x network.
It is known from the above procedure that, in the current mechanism for the handover from the 3GPP PS network to the CDMA2000 1x network, when the UE is handed over from the 3GPP PS network to the CDMA2000 1x network, no mechanism is provided for processing resources in the PS network related to the UE. In addition, the same problem also occurs when the UE is handed over from the PS network to other CS networks similar to the CDMA2000 1x network.